Adam Mann writes at Wired:
Unlike our nice round moon, Mars has two lumpy potato-shaped satellites, named for twin Greek gods who personified fear (Phobos) and terror (Deimos). Phobos is the larger and closer moon to Mars, with an average radius of 11 kilometers. Its companion, Deimos, is seven times less massive and three times farther away from Mars, which is why it appears so much smaller. Both bodies are quite tiny as far as moons go — each is about twice the size of Mount Everest. In fact, Phobos and Deimos are so small and have such weak gravity that going more than 25 mph on their surfaces could launch you into space.